Search

Objective: The perinatal outcomes in second-trimester abdominal circumference (AC) discordant twins are yet to be established. The aim of this study was to ascertain perinatal risks associated with second-trimester AC discordance in monochorionic (MC) twins. Method: We conducted a retrospective study of all MC twin pregnancies over a 7-year period. Intertwin AC discordance at 14–26 gestational weeks was analyzed in relation to Doppler abnormalities, obstetric complications, and perinatal adverse outcomes. Results: A total of 246 MC twin pregnancies were included in the analysis. The smaller twins of second-trimester AC discordant pairs were at increased prevalence of abnormal umbilical artery flow (50% versus 24%, p < .001) and low positive A wave of ductus venous flow (24% versus 9%, p = .002). The second-trimester AC discordant twins were at increased risk of oligohydramnios in smaller twin (OR = 2.44, 95% CI = 1.37–4.32, p < .01), cardiomegaly in larger twin (OR = 2.95, 95% CI = 1.01–8.60, p < .05), birth weight of either twin below the 10th percentile for gestational age (OR = 5.56, 95% CI = 2.67–11.59, p < .001), birth weight discordance > 25% (OR = 9.41, 95% CI = 4.46–19.87, p < .001), IUFD (OR = 3.26, 95% CI = 1.76–6.05, p < .001), and severe neonatal morbidity (OR = 1.83, 95% CI = 1.03–3.26, p < .05). The intact survival rate in discordant and concordant twin pairs was 70% and 89%, respectively (p < .001). Conclusions: Early and increase fetal surveillance of the second-trimester AC discordant twins should be utilized to establish perinatal risks, thus allowing prenatal care to improve. [ABSTRACT FROM AUTHOR]

Objective: The perinatal outcomes in second-trimester abdominal circumference (AC) discordant twins are yet to be established. The aim of this study was to ascertain perinatal risks associated with second-trimester AC discordance in monochorionic (MC) twins.Method: We conducted a retrospective study of all MC twin pregnancies over a 7-year period. Intertwin AC discordance at 14-26 gestational weeks was analyzed in relation to Doppler abnormalities, obstetric complications, and perinatal adverse outcomes.Results: A total of 246 MC twin pregnancies were included in the analysis. The smaller twins of second-trimester AC discordant pairs were at increased prevalence of abnormal umbilical artery flow (50% versus 24%, p < .001) and low positive A wave of ductus venous flow (24% versus 9%, p = .002). The second-trimester AC discordant twins were at increased risk of oligohydramnios in smaller twin (OR = 2.44, 95% CI = 1.37-4.32, p < .01), cardiomegaly in larger twin (OR = 2.95, 95% CI = 1.01-8.60, p < .05), birth weight of either twin below the 10th percentile for gestational age (OR = 5.56, 95% CI = 2.67-11.59, p < .001), birth weight discordance > 25% (OR = 9.41, 95% CI = 4.46-19.87, p < .001), IUFD (OR = 3.26, 95% CI = 1.76-6.05, p < .001), and severe neonatal morbidity (OR = 1.83, 95% CI = 1.03-3.26, p < .05). The intact survival rate in discordant and concordant twin pairs was 70% and 89%, respectively (p < .001).Conclusions: Early and increase fetal surveillance of the second-trimester AC discordant twins should be utilized to establish perinatal risks, thus allowing prenatal care to improve. [ABSTRACT FROM AUTHOR]

Objectives: Twin anemia-polycythemia sequence (TAPS) is associated with increased perinatal morbidity and mortality. Inconsistencies in the diagnostic criteria for TAPS exist, which hinder the ability to establish robust evidence-based management or monitoring protocols. The main aim of this study was to determine, by expert consensus using a Delphi procedure, the key diagnostic features and optimal monitoring approach for TAPS.Methods: A Delphi process was conducted among an international panel of experts on TAPS. Panel members were provided with a list of literature-based parameters for diagnosing and monitoring TAPS. They were asked to rate the importance of the parameters on a five-point Likert scale. Consensus was sought to determine the cut-off values for accepted parameters, as well as parameters used in the monitoring of and assessment of outcome in twin pregnancy complicated by TAPS.Results: A total of 132 experts were approached. Fifty experts joined the first round, of whom 33 (66%) completed all three rounds. There was agreement that the monitoring interval for the development of TAPS should be every 2 weeks and that the severity should be assessed antenatally using a classification system based on middle cerebral artery (MCA) peak systolic velocity (PSV), but there was no agreement on the gestational age at which to start monitoring. Once the diagnosis of TAPS is made, monitoring should be scheduled weekly. For the antenatal diagnosis of TAPS, the combination of MCA-PSV ≥ 1.5 MoM in the anemic twin and ≤ 0.8 MoM in the polycythemic twin was agreed. Alternatively, MCA-PSV discordance ≥ 1 MoM can be used to diagnose TAPS. Postnatally, hemoglobin difference ≥ 8 g/dL and intertwin reticulocyte ratio ≥ 1.7 were agreed criteria for diagnosis of TAPS. There was no agreement on the cut-off of MCA-PSV or its discordance for prenatal intervention. The panel agreed on prioritizing perinatal and long-term survival outcomes in follow-up studies.Conclusions: Consensus-based diagnostic features of TAPS, as well as cut-off values for the parameters involved, were agreed upon by a panel of experts. Future studies are needed to validate these diagnostic features before they can be used in clinical trials of interventions. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Objective: To investigate the antenatal management and outcome in a large international cohort of monochorionic twin pregnancies with spontaneous or post-laser twin anemia-polycythemia sequence (TAPS).Methods: This study analyzed data of monochorionic twin pregnancies diagnosed antenatally with spontaneous or post-laser TAPS in 17 fetal therapy centers, recorded in the TAPS Registry between 2014 and 2019. Antenatal diagnosis of TAPS was based on fetal middle cerebral artery peak systolic velocity > 1.5 multiples of the median (MoM) in the TAPS donor and < 1.0 MoM in the TAPS recipient. The following antenatal management groups were defined: expectant management, delivery within 7 days after diagnosis, intrauterine transfusion (IUT) (with or without partial exchange transfusion (PET)), laser surgery and selective feticide. Cases were assigned to the management groups based on the first treatment that was received after diagnosis of TAPS. The primary outcomes were perinatal mortality and severe neonatal morbidity. The secondary outcome was diagnosis-to-birth interval.Results: In total, 370 monochorionic twin pregnancies were diagnosed antenatally with TAPS during the study period and included in the study. Of these, 31% (n = 113) were managed expectantly, 30% (n = 110) with laser surgery, 19% (n = 70) with IUT (± PET), 12% (n = 43) with delivery, 8% (n = 30) with selective feticide and 1% (n = 4) underwent termination of pregnancy. Perinatal mortality occurred in 17% (39/225) of pregnancies in the expectant-management group, 18% (38/215) in the laser group, 18% (25/140) in the IUT (± PET) group, 10% (9/86) in the delivery group and in 7% (2/30) of the cotwins in the selective-feticide group. The incidence of severe neonatal morbidity was 49% (41/84) in the delivery group, 46% (56/122) in the IUT (± PET) group, 31% (60/193) in the expectant-management group, 31% (57/182) in the laser-surgery group and 25% (7/28) in the selective-feticide group. Median diagnosis-to-birth interval was longest after selective feticide (10.5 (interquartile range (IQR), 4.2-14.9) weeks), followed by laser surgery (9.7 (IQR, 6.6-12.7) weeks), expectant management (7.8 (IQR, 3.8-14.4) weeks), IUT (± PET) (4.0 (IQR, 2.0-6.9) weeks) and delivery (0.3 (IQR, 0.0-0.5) weeks). Treatment choice for TAPS varied greatly within and between the 17 fetal therapy centers.Conclusions: Antenatal treatment for TAPS differs considerably amongst fetal therapy centers. Perinatal mortality and morbidity were high in all management groups. Prolongation of pregnancy was best achieved by expectant management, treatment by laser surgery or selective feticide. © 2020 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of the International Society of Ultrasound in Obstetrics and Gynecology. [ABSTRACT FROM AUTHOR]

Objective: Selective fetal growth restriction (sFGR) occurs in monochorionic twin pregnancies when unequal placental sharing leads to restriction in the growth of just one twin. Management options include laser separation of the fetal circulations, selective reduction or expectant management, but what constitutes the best treatment is not yet known. New trials in this area are urgently needed but, in this rare and complex group, maximizing the relevance and utility of clinical research design and outputs is paramount. A core outcome set ensures standardized outcome collection and reporting in future research. The objective of this study was to develop a core outcome set for studies evaluating treatments for sFGR in monochorionic twins.Methods: An international steering group of clinicians, researchers and patients with experience of sFGR was established to oversee the process of development of a core outcome set for studies investigating the management of sFGR. Outcomes reported in the literature were identified through a systematic review and informed the design of a three-round Delphi survey. Clinicians, researchers, and patients and family representatives participated in the survey. Outcomes were scored on a Likert scale from 1 (limited importance for making a decision) to 9 (critical for making a decision). Consensus was defined a priori as a Likert score of ≥ 8 in the third round of the Delphi survey. Participants were then invited to take part in an international meeting of stakeholders in which the modified nominal group technique was used to consider the consensus outcomes and agree on a final core outcome set.Results: Ninety-six outcomes were identified from 39 studies in the systematic review. One hundred and three participants from 23 countries completed the first round of the Delphi survey, of whom 88 completed all three rounds. Twenty-nine outcomes met the a priori criteria for consensus and, along with six additional outcomes, were prioritized in a consensus development meeting, using the modified nominal group technique. Twenty-five stakeholders participated in this meeting, including researchers (n = 3), fetal medicine specialists (n = 3), obstetricians (n = 2), neonatologists (n = 3), midwives (n = 4), parents and family members (n = 6), patient group representatives (n = 3), and a sonographer. Eleven core outcomes were agreed upon. These were live birth, gestational age at birth, birth weight, intertwin birth-weight discordance, death of surviving twin after death of cotwin, loss during pregnancy or before final hospital discharge, parental stress, procedure-related adverse maternal outcome, length of neonatal stay in hospital, neurological abnormality on postnatal imaging and childhood disability.Conclusions: This core outcome set for studies investigating the management of sFGR represents the consensus of a large and diverse group of international collaborators. Use of these outcomes in future trials should help to increase the clinical relevance of research on this condition. Consensus agreement on core outcome definitions and measures is now required. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Objectives: To explore the outcome of monochorionic monoamniotic (MCMA) twin pregnancies affected by twin-to-twin transfusion syndrome (TTTS).Methods: MEDLINE and EMBASE databases were searched for studies reporting the outcome of MCMA twin pregnancies complicated by TTTS. The primary outcome was intrauterine death (IUD); secondary outcomes were miscarriage, single IUD, double IUD, neonatal death (NND), perinatal death (PND), survival of at least one twin, survival of both twins and preterm birth (PTB) before 32 weeks' gestation. Outcomes were assessed in MCMA twins affected by TTTS not undergoing intervention and in those treated with amniodrainage, laser therapy or cord occlusion. Subgroup analysis was performed including cases diagnosed before 24 weeks. Random-effects meta-analysis of proportions was used to analyze the data.Results: Fifteen cohort studies, including 888 MCMA twin pregnancies, of which 44 were affected by TTTS, were included in the review. There was no randomized trial comparing the different management options in MCMA twin pregnancies complicated by TTTS. In cases not undergoing intervention, miscarriage occurred in 11.0% of fetuses, while the incidence of IUD, NND and PND was 25.2%, 12.2% and 31.2%, respectively. PTB complicated 50.5% of these pregnancies. In cases treated by laser surgery, the incidence of miscarriage, IUD, NND and PND was 19.6%, 27.4%, 7.4% and 35.9%, respectively, and the incidence of PTB before 32 weeks' gestation was 64.9%. In cases treated with amniodrainage, the incidence of IUD, NND and PND was 31.3%, 13.5% and 45.7% respectively, and PTB complicated 76.2% of these pregnancies. Analysis of cases undergoing cord occlusion was affected by the very small number of included cases. Miscarriage occurred in 19.2%, while there was no case of IUD or NND of the surviving twin. PTB before 32 weeks occurred in 50.0% of these cases.Conclusions: MCMA twin pregnancies complicated by TTTS are at high risk of perinatal mortality and PTB. Further studies are needed in order to elucidate the optimal type of prenatal treatment in these pregnancies. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Objectives: To evaluate the long-term neurodevelopmental and behavioral outcomes in surviving infants of pregnancies with spontaneous twin anemia-polycythemia sequence (TAPS), to compare outcome between donors and recipients, and to investigate potential risk factors for neurodevelopmental impairment (NDI).Methods: This was a retrospective study of a consecutive cohort of spontaneous-TAPS survivors delivered between 2005 and 2017 at the Leiden University Medical Center, The Netherlands. Neurological, motor, cognitive and behavioral development were assessed at a median age of 4 years. The primary outcome was NDI, which was a composite outcome of cerebral palsy, deafness, blindness and motor and/or cognitive delay. NDI was subdivided into two grades of severity: mild-to-moderate and severe NDI. Outcome was compared between surviving donor and recipient twins. Logistic regression analysis was used to assess risk factors for NDI.Results: Forty-nine twin pregnancies complicated by spontaneous TAPS were eligible for inclusion. The perinatal survival rate was 83% (81/98) of twins. Neurodevelopmental assessment was performed in 91% (74/81) of surviving twins. NDI occurred in 30% (22/74) of TAPS survivors, and was found more often in donors (44%; 15/34) than in recipients (18%; 7/40) (odds ratio (OR), 4.1; 95% CI, 1.8-9.1; P = 0.001). Severe NDI was detected in 9% (7/74) of survivors and was higher in donors compared with recipients (18% (6/34) vs 3% (1/40)), although the difference did not reach statistical significance; P = 0.056). Donors demonstrated lower cognitive scores compared with recipients (P = 0.011). Bilateral deafness was identified in 15% (5/34) of donors compared with 0% (0/40) of recipients (P = 0.056). Parental concern regarding development was reported more often for donor than for recipient twins (P = 0.001). On multivariate analysis, independent risk factors for NDI were gestational age at delivery (OR, 0.7; 95% CI, 0.5-0.9; P = 0.003) and severe anemia (OR, 6.4; 95% CI, 2.4-17.0; P < 0.001).Conclusion: Surviving donor twins of pregnancies complicated by spontaneous TAPS have four-fold higher odds of NDI compared with recipient cotwins, are at increased risk of cognitive delay and have a high rate of deafness. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Objectives: Twin pregnancy complicated by selective fetal growth restriction (sFGR) is associated with increased perinatal mortality and morbidity. Inconsistencies in the diagnostic criteria for sFGR employed in existing studies hinder the ability to compare or combine their findings. It is therefore challenging to establish robust evidence-based management or monitoring pathways for these pregnancies. The main aim of this study was to determine, by expert consensus using a Delphi procedure, the key diagnostic features of and the essential reporting parameters in sFGR.Methods: A Delphi process was conducted among an international panel of experts in sFGR in twin pregnancy. Panel members were provided with a list of literature-based parameters for diagnosing sFGR and were asked to rate their importance on a five-point Likert scale. Parameters were described as solitary (sufficient to diagnose sFGR, even if all other parameters are normal) or contributory (those that require other abnormal parameter(s) to be present for the diagnosis of sFGR). Consensus was sought to determine the cut-off values for accepted parameters, as well as parameters used in the monitoring, management and assessment of outcome of twin pregnancy complicated by sFGR. The questions were presented in two separate categories according to chorionicity.Results: A total of 72 experts were approached, of whom 60 agreed to participate and entered the first round; 48 (80%) completed all four rounds. For the definition of sFGR irrespective of chorionicity, one solitary parameter (estimated fetal weight (EFW) of one twin < 3rd centile) was agreed. For monochorionic twin pregnancy, at least two out of four contributory parameters (EFW of one twin < 10th centile, abdominal circumference of one twin < 10th centile, EFW discordance of ≥ 25%, and umbilical artery pulsatility index of the smaller twin > 95th centile) were agreed. For sFGR in dichorionic twin pregnancy, at least two out of three contributory parameters (EFW of one twin < 10th centile, EFW discordance of ≥ 25%, and umbilical artery pulsatility index of the smaller twin > 95th centile) were agreed.Conclusions: Consensus-based diagnostic features of sFGR in both monochorionic and dichorionic twin pregnancies, as well as cut-off values for the parameters involved, were agreed upon by a panel of experts. Future studies are needed to validate these diagnostic features before they can be used in clinical trials of interventions. Copyright © 2018 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Objective: The optimal outcome after fetoscopic laser surgery (FLS) for twin-twin transfusion syndrome (TTTS) depends on the successful ablation of all placental anastomoses. The objective of this study was to determine the incidence of and risk factors for recurrent TTTS (rTTTS) or twin anemia-polycythemia sequence (TAPS) after FLS, focusing on the impact of cannula diameter.Methods: This was a secondary analysis of data collected prospectively at two centers from 666 consecutive patients undergoing FLS for TTTS. The main outcomes were rTTTS and TAPS following FLS. Variables assessed included gestational age at intervention, stage of disease, recipient maximum vertical pocket, anterior placenta, number of anastomoses ablated, cannula diameter/operative scopes and use of the Solomon technique. Cannula diameter and corresponding scopes used were as follows: 8 Fr and 1.3 mm/0°; 9 Fr and 2.7 mm/0°; 10 Fr and 3 mm/0°; or 12 Fr and 3.3-3.7 mm/30-70°. Cannula diameter was used as a surrogate for scopes during analysis. Multivariate logistic regression analysis was performed to identify risk factors associated with rTTTS or TAPS after FLS; 'center' was considered an independent variable to account for variations in practice. In a nested cohort of pregnancies in which both fetuses survived, placental dye injection was performed in 315 placentae. Multivariate logistic regression analysis was performed to evaluate variables associated with the presence of residual anastomoses.Results: rTTTS or TAPS occurred in 61 (9%) cases following FLS (rTTTS in eight (1%) and TAPS in 53 (8%)). Factors associated significantly with the risk of rTTTS/TAPS on multivariate analysis were cannula diameter (when an 8-Fr, 9-Fr, 10-Fr or 12-Fr cannula was used, there was rTTTS/TAPS in 24%, 13%, 2% or 0.8% of cases, respectively (P < 0.001)) and use of the Solomon technique (rTTTS/TAPS occurred in 4.2% of those in which it was used vs 18.1% in those in which it was not (P < 0.001)). Only use of the Solomon technique was associated significantly with no residual anastomoses found after delivery.Conclusions: Following FLS for TTTS, a lower incidence of rTTTS/TAPS was seen when the Solomon technique was used, as well as when a 10-Fr or 12-Fr cannula was used. A lower complication rate may be due to the use of a scope with better optics during placental mapping. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

We conclude that currently available data do not indicate a major benefit of IUT for survival or brain injury risk in the anemic cotwin after SFD. Intrauterine transfusion (IUT) can correct successfully fetal anemia in the surviving cotwin after SFD; however, it is unclear if this treatment is beneficial for the long-term prognosis. Single fetal demise (SFD) in monochorionic (MC) twin pregnancy is associated with high risk of perinatal morbidity and mortality of the cotwin, including injury to the brain and other organs due to acute retrograde exsanguination1. [Extracted from the article]

ABSTRACT Objectives Severe right ventricular outflow tract obstruction ( RVOTO) is a potential complication in recipient twins of twin-to-twin transfusion syndrome ( TTTS) that requires postnatal follow-up or treatment. We aimed to evaluate pregnancy characteristics of neonates with RVOTO from complicated monochorionic twin pregnancies, determine the incidence of RVOTO in TTTS cases and construct a prediction model for its development. Methods This was an observational cohort study of all complicated monochorionic twin pregnancies with a postnatal diagnosis of RVOTO examined at our center. Cases were referred for evaluation of the need for fetal therapy or intervention because of TTTS, selective intrauterine growth restriction ( sIUGR) or multiple congenital malformations in one of the twins. Ultrasound data were retrieved from our monochorionic twin database. Among liveborn TTTS recipients treated prenatally with laser therapy, those with RVOTO were compared with those without RVOTO (controls). We describe four additional cases with RVOTO that were not TTTS recipients. Results A total of 485 twin pregnancies received laser therapy for TTTS during the study period. RVOTO was diagnosed in 3% (11/368) of liveborn TTTS recipients, of whom two showed mild Ebstein's anomaly. Before laser therapy, pericardial effusion was seen in 45% (5/11) of RVOTO cases ( P < 0.01) and abnormal A-wave in the ductus venosus ( DV) in 73% (8/11) ( P = 0.03), significantly higher proportions than in controls. Mean gestational age at laser therapy was 17 + 3 weeks in RVOTO cases compared with 20 + 3 weeks in controls ( P = 0.03). A prediction model for RVOTO was constructed incorporating these three significant variables. One TTTS donor had RVOTO after the development of transient hydrops following laser therapy. Three larger twins in pregnancies complicated by sIUGR developed RVOTO, the onset of which was detectable early in the second trimester. Conclusions RVOTO occurs in TTTS recipient twins but can also develop in TTTS donors and larger twins of pregnancies complicated by sIUGR. Abnormal flow in the DV, pericardial effusion and early gestational age at onset of TTTS are predictors of RVOTO in TTTS recipients, which suggests increased vulnerability to hemodynamic imbalances in the fetal heart in early pregnancy. These findings could guide diagnostic follow-up protocols after TTTS treatment. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

The article offers information on the clinical practice guidelines of the scientific organization International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) in addressing the importance of ultrasound assessment in twin pregnancy. Topics discussed include the twin-to-twin transfusion syndrome (TTTS), how to determine chorionicity/amnionicity in twin pregnancy, and the management of fetal growth restriction (FGR).

Objective: To evaluate the diagnostic accuracy of middle cerebral artery peak systolic velocity (MCA-PSV) Doppler measurements in prediction of hemoglobin levels in twin anemia-polycythemia sequence (TAPS).Methods: This study involved a consecutive cohort comprising monochorionic twin pregnancies complicated by TAPS managed at three European fetal medicine centers between 2005 and 2013. The accuracy of MCA-PSV, measured immediately prior to fetal hemoglobin (Hb) measurement by fetal or cord blood sampling, for prediction of anemia and polycythemia was assessed using 2 × 2 tables.Results: A total of 116 measurements (74 recorded in donors and 42 in recipients) from 43 twin pregnancies complicated by TAPS were available for analysis. MCA-PSV multiples of the median (MoM) values correlated well with Hb levels (r = - 0.86; P < 0.001). The sensitivity of MCA-PSV ≥ 1.5 MoM to predict severe anemia (Hb deficit > 5 SD below the mean) in TAPS donors was 94% (95% CI, 85-98%); specificity was 74% (95% CI, 62-83%); positive and negative predictive values were 76% (95% CI, 65-85%) and 94% (95% CI, 83-98%), respectively. The sensitivity of MCA-PSV ≤ 1.0 MoM to predict polycythemia (Hb level > 5 SD above the mean) in TAPS recipients was 97% (95% CI, 87-99%); specificity was 96% (95% CI, 89-99%); positive and negative predictive values were 93% (95% CI, 81-97%) and 99% (95% CI, 93-100%), respectively.Conclusion: MCA-PSV measurement has high diagnostic accuracy for predicting abnormal Hb levels in fetuses with TAPS. [ABSTRACT FROM AUTHOR]

ABSTRACT Objective To determine, by expert consensus, the essential substeps of fetoscopic laser surgery ( FLS) for twin-twin transfusion syndrome ( TTTS) that could be used to create an authority-based curriculum for training in this procedure among fetal medicine specialists. Methods A Delphi survey was conducted among an international panel of experts ( n = 98) in FLS. Experts rated the substeps of FLS on a five-point Likert-type scale to indicate whether they considered them to be essential, and were able to comment on each substep, using a dedicated online platform accessed by the invited tertiary care facilities that specialize in fetal therapy. Responses were returned to the panel until consensus was reached (Cronbach's α ≥ 0.80). All substeps that were rated ≥ 4 by 80% of the experts were included in the evaluation instrument. Results After the first iteration of the Delphi procedure, a response rate of 74% (73/98) was reached, and in the second and third iterations response rates of 90% (66/73) and 81% (59/73) were reached, respectively. Among a total of 81 substeps rated in the first round, 21 substeps had to be re-rated in the second round. Finally, from the initial list of substeps, 55 were agreed by experts to be essential. In the third round, the 18 categorized substeps were ranked in order of importance, with 'coagulation of all anastomoses that cross the equator' and 'determination of fetoscope insertion site' as the most important. Conclusions A total of 55 substeps of FLS for TTTS were defined by a panel of experts to be essential in the procedure. This list is the first authority-based evidence to be used in the development of a final training model for future fetal surgeons. Copyright © 2014 ISUOG. Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]